JP2001073072A - Carbo-nitrided parts excellent in pitching resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide parts for machine structure increased in softening resistance and exhibiting excellent pitching resistance by composing the same of steel having a specified compsn., specifying the content of [Si+Mn+Mo], allowing it to have a surface hardened layer subjected to carbo-nitriding or quenching and tempering treatment after carbo-nitriding and respectively specifying the C content [Cs] and N content [Ns] from the surface to a specified depth and R value. SOLUTION: This carbo-nitrided parts are composed of steel contg., by mass, 0.15 to 0.25% C, 0.40 to 0.9% Si, 0.05 to 0.7% Mn, 1.25 to 2.5% Cr, 0.35 to 1% Mo, 0.02 to 0.6% Al and 0.007 to 0.015% N, and the balance substantially Fe, have [Si+Mn+Mo] contents of 1.0 to 2.20% and have surface hardened layers subjected to carbo-nitriding or quenching and tempering after carbo-nitriding, in which the C content [Cs] from the surface to 0.1 mm is controlled to >=0.7%, the N content [Ns] to 0.6 to 2.0%, and R value obtd. by the formula is controlled to >=7.5.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbonitrided part having a high surface hardness by carbonitriding, good softening resistance and excellent pitting resistance. It can be effectively used as high-strength parts for obtaining gears and shafts used in construction machines and other various industrial machines.

[0002]

2. Description of the Related Art Among mechanical parts used in automobiles, construction machines, and various other industrial machines, parts that require particularly strong fatigue resistance and wear resistance are usually case hardening for machine structures. A steel that has been processed into a desired shape and then subjected to a surface hardening treatment is used.

[0003] As such a surface hardening method, carburizing, high-frequency heating, coating treatment, and the like are known. However, parts requiring good machinability and a high level of base material toughness are mainly used for low carbon skin. A method of increasing the surface carbon concentration to about 0.7% by using carburized steel and gas carburizing has been adopted. However, as the performance of the above-mentioned machinery has been further enhanced, the operating conditions have become more severe, and especially in sliding parts such as gears for transmitting power of automobiles and the like, more excellent softening resistance. Is strongly desired, and carbonitriding is attracting attention instead of gas carburizing.

[0004] Softening resistance is an important factor for pitting resistance under high load conditions, and it has been reported in many literatures that it has a direct relationship with pitting occurrence life (particularly, No. 9-296250).

According to the carbonitriding treatment, it has been confirmed that not only the quenchability is improved but also the softening resistance is enhanced by the action of invading nitrogen.
Japanese Patent No. 0438 discloses a method of manufacturing a machine part having improved quenching properties of a matrix and excellent pitting resistance and bending fatigue strength by controlling the composition of steel materials and the conditions of carbonitriding. ing. However, in these conventional carbonitriding treatments, since the nitrogen concentration in the surface layer, which is the starting point of pitting, is not sufficiently increased, it cannot always be said that the recent demand for higher strength can be satisfied.

Therefore, it is desired to develop a carbonitriding method capable of further increasing the amount of invading nitrogen. However, when the nitrogen concentration in the surface layer is increased, not only the amount of residual γ increases, but also the amount of CrN precipitated at the former γ grain boundary. As the Cr content increases, the Cr concentration in the peripheral portion decreases, so that an incompletely quenched layer tends to appear, and as a result, it is difficult to obtain a sufficient surface hardness. Further, for the purpose of precipitating and dispersing V-based carbides and carbonitrides, which are considered to greatly contribute to improvement in softening resistance,
(Japanese Patent Application Laid-Open No. Hei 8-120438) is also known, but in addition to the fact that V is expensive, if the amount of V is increased, the hardenability of the hardened layer matrix is further reduced, and It is feared that the hardness is lowered.

In addition, since the effects of the Si content and Mn content of the steel material on productivity, workability, etc., such as cold pressability and machinability, are not sufficiently taken into consideration, from the viewpoint of improving productivity. Then, further improvement is desired.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and an object of the present invention is to increase the softening resistance without extremely reducing the productivity of steel materials. It is an object of the present invention to provide a carbonitriding or carbonitriding (hereinafter simply referred to as carbonitriding) part for a machine structure which exhibits excellent pitting resistance.

[0009]

The carbonitrided part according to the present invention, which has solved the above-mentioned problems, has a carbon content of 0.15.
0.25% (mass%, the same applies hereinafter), Si: 0.40
0.9%, Mn: 0.05-0.7%, Cr: 1.25
-2.5%, Mo: 0.35-1%, Al: 0.02-
Consisted of steel containing 0.06% and N: 0.007 to 0.015%, with the balance being substantially Fe,
Mn + Mo] is 1.0 to 2.20%, and has a surface hardened layer that is quenched or tempered after carbonitriding or carbonitriding, and has a C content [Cs] of 0.7 mm from the surface to 0.7 mm.
% Or more, the N content [Ns] is 0.6 to 2.0%, and the R value obtained by the following formula (I) is 7.5 or more. R value = 1.11 × [Cs] + 1.25 × [Ns] + 1.89 × Si + 1.22 × Mn + 0.67 × Mo + 3.94 (I)

In the carbonitrided parts of the present invention, those having 0.05% of Si-based carbonitride in the surface hardened layer are parts for machine structure showing more excellent pitting resistance and wear resistance. In addition, after the carbonitriding treatment or after the quenching and tempering treatment, the shot peening treatment is performed to increase the surface layer hardness and to give the residual stress, and the wear resistance and the pitting resistance as the parts for machine structure are improved. It is effective in further raising.

When the steel according to the present invention further contains Cu: 1% or less and / or Ni: 1% or less (both not including 0%) as other elements, the corrosion resistance of the carbonitrided layer is improved. And Nb: 1% or less, Ti: 1% or less, and B: 0.1% or less.
% Or less (both do not include 0%) can reduce austenite crystal grains and carbonitrides, which is effective in improving toughness. In addition, S, C
The use of steel containing at least one element selected from the group consisting of a, Zr, Sb, Pb and Bi in an amount of 0.1% or less, respectively, further enhances the toughness of the base material and the machinability. The use of a steel material containing these elements is also recommended as a preferred embodiment.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, in the present invention, the composition of the steel material to be used is specified and the depth of the surface hardened layer formed by carbonitriding is 0.1 m in the surface layer.
C amount [Cs] and N amount [Ns] up to m,
By specifying the R value obtained by (I), especially the softening resistance is improved to increase the pitting resistance. The reasons for defining the above requirements are as follows.

First, the reasons for determining the chemical composition of the steel used in the present invention are as follows.

C: 0.15 to 0.25% C is an element indispensable for imparting a predetermined core hardness to a carbonitrided part and ensuring an effective hardening depth. 0.15 to effectively demonstrate
% Or more. However, if the C content is too large, the toughness, machinability, and cold workability of the steel material decrease.
The upper limit is set to 0.25%. A more preferred lower limit of the C content is 0.17%, and a more preferred upper limit is 0.22%.

Si: 0.40 to 0.9% Si is an element which forms carbonitride and greatly contributes to the improvement of the softening resistance of the surface carbonitride layer. In addition to suppressing precipitation, it is also an effective element for increasing the wear resistance by spheroidizing carbonitrides. It also has the effect of hardening the matrix of the carbonitrided layer, and in order to exhibit these effects effectively, it must be contained at 0.40% or more. However, if the content is too large, the carbonitriding property of the steel is impaired, and the toughness and machinability of the part are remarkably deteriorated. Therefore, the content must be suppressed to 0.9% or less. The preferable upper limit of the amount of Si is 0.8%.

Mn: 0.05-0.7% Mn also contributes significantly to the improvement of softening resistance by forming carbonitrides, and also acts as a deoxidizing component during melting.
Furthermore, it also has an effective effect on improving hardenability and cutting workability by forming MnS. In order to effectively exert the effect of Mn, it is necessary to contain at least 0.05% or more. However, if it is too much, it adversely affects forgeability and machinability, and the amount of retained austenite in the surface layer becomes excessive. And instead lowers the surface hardness.
It must be kept below 7%. The lower limit of the Mn content is more preferably 0.2% in consideration of such advantages and disadvantages.

Cr: 1.25 to 2.5% Cr enhances the hardenability of the base material and imparts a stable hardened layer depth and a required core hardness, so that it can be used as a structural member such as a gear. It is an important component in securing the mechanical strength and fatigue strength, and also in improving the tempering softening resistance of the base of the surface hardened layer to improve the pitting resistance, and must be contained at least 1.25% or more. However, if the amount of Cr becomes too large, a large amount of CrN precipitates at the old γ grain boundary during carbonitriding as in the case of increasing the amount of intruding nitrogen in the conventional example, and the hardenability decreases due to a decrease in the amount of solute Cr in austenite. Reduces the surface hardness. If the content is too large, the carbonitriding property is impaired and the machinability is adversely affected. Therefore, the content must be suppressed to 2.5% or less. The lower limit of the more preferable Cr content is 1.4%, and the more preferable upper limit is 2.2%.

Mo: 0.35% to 1% Mo increases the amount of invading nitrogen to increase the Cr content.
Even if the precipitation amount of N increases, it has a very important effect in maintaining the hardenability around it at a high level, and further has an effect of promoting the formation of carbonitride and miniaturization thereof. ing. That is, the amount of carbonitriding increases as the Mo content increases, and since the composite carbonitride is hard, it effectively acts to increase the hardness of the carbonitriding layer. By these actions, Mo contributes to the reduction of an abnormal layer such as an incompletely quenched structure on the surface of the carbonitrided layer and the improvement of the strength inside the carbonitrided layer. In order to effectively exert such an effect, the content must be 0.35% or more. However, if the Mo content is too large, the toughness and machinability as a mechanical component are reduced, and the amount of retained austenite in the surface layer is excessive, thereby decreasing the surface hardness.
It must be kept below 1%. A more preferred lower limit of the Mo amount is 0.4%, and a more preferred upper limit is 0.9%.

Si + Mn + Mo: 1.0 to 2.20% Si, Mn, and Mo each have the above-described effects, but all of them increase the tempering softening resistance in a quasi-high temperature range to increase the pitting resistance. Works effectively to improve However, on the other hand, if the content is excessive, the toughness and productivity as mechanical parts are impaired. That is, considering the productivity of steel material such as cold pressure and machinability, Si, Mn,
It is important to contain Mo in a well-balanced manner. To ensure the excellent softening resistance intended in the present invention without extremely lowering the productivity as a constituent material, [Si
+ Mn + Mo] from 1.0 to 2.20%
Must be within the range. [Si + Mn + M
o] The more preferable range of the total content is 1.2 to 1.
8%.

Al: 0.02 to 0.06% Al acts as a deoxidizing agent, and is also an element effective in generating AlN during heat treatment and refining crystal grains after quenching to enhance toughness. In order to effectively exert these effects, the content must be 0.02% or more. But A
If the l content is excessive, the crystal grains that have been refined at an angle will aggregate and cause the growth of the crystal grains. Therefore, the content must be suppressed to 0.06% or less. From such a viewpoint, the more preferable upper limit of the Al content is 0.04%.

N: 0.007 to 0.015% N combines with Al to form AlN, and has an effect of making austenite crystal grains fine, and thus contributes to an improvement in pitting life. Such an effect is effectively exerted by setting the N content to 0.007% or more, but since these effects are saturated at 0.015%, the upper limit is set to 0.015%. A more preferred lower limit of the N content is 0.008%, and a more preferred upper limit is 0.012%.
It is.

The essential constituent elements of the steel material used in the present invention are as described above, and the balance is substantially Fe. Here, “substantially” means the effects and effects of the respective component elements described above,
In addition, other elements are positively contained or inevitably mixed with the steel material after carbonitriding or after further quenching and tempering, so long as the properties of the carbonitrided parts obtained are not impaired. Means that elements that can be mixed are allowed. Examples of effective elements that can be positively contained include the following.

Cu: 1% or less and / or Ni: 1%
The following elements (both do not include 0%) are elements that contribute to the improvement of the corrosion resistance of the carbonitrided layer by forming a solid solution in the matrix of the carbonitrided layer.
i also exerts a toughness improving action. However, when the amount of Cu is 1
%, The hot workability as a part decreases, and N
If the i content exceeds 1%, the amount of retained austenite increases and the surface hardness may decrease.

At least one element selected from the group consisting of Nb: 1% or less, Ti: 1% or less, and B: 0.1% or less (both do not include 0%). Has the effect of forming a product and refining austenite crystal grains during heating,
Further, Nb is fine M
_It also has an effect of suppressing carbide transformation from ₇ (C, N) ₃ to coarse M ₂₃ (C, N) ₆ and preventing deterioration of fatigue fracture resistance. However, such an effect of Nb is saturated at 1%, and when the Ti amount and the B amount exceed 1% and 0.1%, respectively, a reduction in toughness and a reduction in fatigue strength due to excessive generation of carbonitrides are caused.

At least one element selected from the group consisting of S, Ca, Zr, Sb, Pb and Bi: not more than 0.1% each S, Ca, Zr, Sb, Pb and Bi are all machinable Is an element effective in improving the toughness, and Zr also contributes to the improvement in toughness. However, if the S content exceeds 0.1%, the toughness is reduced, and MnS generated by bonding with the Mn is reduced.
Is a starting point of destruction and shortens the pitting life. In addition, Sb, Pb, and Bi not only saturate the effect even if they are added excessively, but also serve as a generation source of large nonmetallic inclusions, become a starting point of surface breakdown, and reduce pitting life.
Further, Ca is generated as CaO around Al ₂ O ₃ and has an effect of increasing machinability without deteriorating pitting resistance, but the effect is saturated at 0.1%. Zr also suppresses deformation of MnS during hot rolling, and has an effect of increasing machinability without deteriorating pitting resistance by granulating MnS, but when the content is too large, , ZrO _{2 and} other non-metallic inclusions are formed in large amounts and adversely affect the pitting resistance. Therefore, they must be suppressed to 0.1% or less.

The chemical composition of the steel material according to the present invention is as described above. After the steel material satisfying those requirements is processed into a predetermined component shape, after carbonitriding, or further quenching / tempering, In the present invention, a predetermined carbonitrided layer is formed on the surface of the component. In the present invention, the C content [C at a depth of 0.1 mm from the surface constituting the carbonitrided layer is [C
s] is 0.7% or more, and the N amount [Ns] at the same depth position is set to 0.1%.
By setting the R value determined by the formula (I) to 7.5 or more by 6 to 2.0%, the tempering softening resistance of the surface hardened layer is increased, and the pitting resistance is dramatically increased. The main reasons for setting these requirements are as follows.

[Cs]: 0.7% or more, [Ns]: 0.
6 to 2.0% [Cs] and [Ns] are indices indicating the amount of carburization and the amount of nitriding in the carbonitrided surface layer,
The [Cs] value is an important requirement mainly for securing the hardness of the surface hardened layer. In order to give sufficient strength and surface hardness to the carbonitrided layer, the [Cs] must be 0.7% or more. Must. [Ns] is an important requirement for increasing the tempering softening resistance of the surface hardened layer mainly in the quasi-high temperature range after quenching. In the present invention, particularly, the contribution of Si-based and Fe-based carbonitrides Is big. The [Ns] is 0.6
%, Si-based carbonitrides contributing to the improvement of temper softening resistance are not precipitated. On the other hand, when the [Ns] exceeds 2.0%, the amount of retained austenite becomes excessive. Or an abnormal structure such as an incompletely quenched structure is likely to appear, and the surface hardness is extremely reduced.

R value: 7.5 or more As described above, the chemical composition of the steel material used, particularly Si,
The contents of Mn and Mo and the values of [Cs] and [Ns] in the surface hardened layer formed by the carbonitriding treatment greatly affect the amount of carbonitride deposited and the form of carbonitride, and satisfy the above-mentioned requirements. This increases the tempering softening resistance particularly in the quasi-high temperature range to provide excellent pitting resistance. However, in order to more reliably exert such an effect, the R value obtained by the above formula (I) is 7.5 or more. Is extremely important.

That is, the R value is highly correlated with the surface hardness after softening due to the atmosphere in which the parts are used or the heat generated during the use (such as frictional heat), and the pitting characteristics are sufficient to withstand recent severe use conditions. In order to secure this, it is an essential requirement that the R value be 7.5 or more, as will be apparent from the examples described later. As is clear from the above formula (I), the R value includes [Cs], [Ns], Si, M in the surface hardened layer.
The contents of n and Mo affect each other.
The effect of the amount of i is the largest, and this tendency is consistent with the fact that the presence of the Si-based carbonitride, which will be described later in the present invention, has a significant effect on the improvement of pitting resistance. I can say.

The content of Si-based carbonitride: 0.05% or more In the present invention, the essential components required for the surface-hardened layer are the above [Cs], [Ns] and R value. Another requirement is that the amount of Si-based carbonitride is 0.0
Carbonitrided parts with 5% or more, more preferably 0.1% or more, exhibit more excellent pitting resistance and wear resistance. The amount of Si-based carbonitride in such a hardened surface layer should be 0.40% or more in terms of the Si content in the steel used, and the carbonitriding conditions should be properly controlled.
This can be achieved by securing 0.6% or more in the [Ns] value of the surface hardened layer.

As described above, the carbonitrided part of the present invention not only specifies the component composition of the steel material, but also determines the [Cs], [Ns], and R values at a depth of 0.1 mm from the surface, and more preferably Si. By defining the amount of carbonitride, it provides excellent surface hardness and abrasion resistance, as well as excellent softening resistance to give excellent pitting resistance, and its production method is not particularly limited. A typical production method is as follows.

First, as a steel material to be used, a steel material satisfying the above-mentioned composition is used, and after processing into a predetermined part shape, carburizing and nitriding are performed sequentially or simultaneously to perform carbonitriding. The specific method of carbonitriding is not particularly limited, and a normal gas carbonitriding method, a plasma carbonitriding method, or the like may be employed. Although the condition is not particularly limited, typical methods when employing the gas carbonitriding method, CO ₂ containing gas as the carburizing gas, a method of using NH ₃ containing gas as a nitriding gas, the amount of carbonitriding,
The concentration may be adjusted according to the CO ₂ concentration and the NH ₃ concentration in the carburizing and / or nitriding gas, their gas flow rates, temperatures, and the like.

The carbonitriding treatment may be performed in one step,
Alternatively, two or more stages of multiple-stage treatment may be employed depending on the degree of carbonitriding, but usually one stage of carburizing or carbonitriding is performed at about 900 ° C., and then 850 ° C.
The method of performing the carbonitriding treatment in the second stage before and after is preferably adopted.

After carbonitriding, oil quenching, tempering at about 170 ° C., and air cooling are generally employed.

The conditions of the carbonitriding treatment and the quenching / tempering treatment are not limited to the characteristics of the present invention, and may be arbitrarily changed according to the type of steel material to be used and the degree of carbonitriding of the surface hardened layer. Can be implemented.

The carbonitrided part or the quenched or tempered part to form a predetermined surface hardened layer is subjected to a finishing surface treatment as necessary to obtain a carbonitrided part.・
After the tempering treatment, the surface of the surface hardened layer forming member is shot peened (preferably 0.4 m in arc height).
It is recommended to further increase the surface hardness and the residual stress of the surface layer portion and further increase the wear resistance and the pitting resistance as a preferred embodiment when the present invention is put to practical use. Is done.

The carbonitrided part of the present invention thus obtained has high surface hardness and abrasion resistance and, in particular, has excellent resistance to softening and has excellent pitting resistance. In addition, it can be widely and effectively utilized as high-strength parts for obtaining sliding parts such as shafts and gears used in various industrial machines, bearings and the like.

[0038]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and the present invention is not limited thereto. Modifications may be made and all of them are included in the technical scope of the present invention.

Test steels having the chemical components shown in Table 1 were melted in a small furnace, and subjected to hot forging followed by normalizing to form round bar test specimens having a diameter of 10 mm × length 100 mm and a diameter of 26 mm × length 100 mm. Machined. Each of the obtained test pieces was subjected to carbonitriding treatments (A) and (B) shown in FIGS. The carbon potential during carbonitriding was adjusted by changing the composition of the carbonitriding gas, and the nitrogen potential was adjusted by changing the flow rate of ammonia.

After carbonitriding, the diameter is 10 mm and the length is 10
0.8 mmA (arc height) for a 0 mm round bar specimen
After the shot peening process, and then performed at 300 ° C for 1
After performing tempering treatment for 80 minutes, 0.10m from the surface
The hardness at the m position was measured by an automatic micro Vickers hardness tester.

The [Cs] and [Ns] values of each test piece were determined by sampling three layers of cutting powder from the surface of a round bar test piece having a diameter of 26 mm x a length of 100 mm to a depth of 0.15 mm every 0.05 mm. Each was subjected to chemical analysis, and the average value of the second and third layers was determined as a value at the 0.10 mm position. Also,
The amount of Si-based carbonitride in the surface hardened layer was determined by electropolishing the surface of each test piece to a depth of 0.1 mm and analyzing the polished surface by an X-ray diffraction method.

The steel type q is JIS standard "SCM42
0 "steel, and the steel type r is" SCr420 "steel.
Table 2 shows the results.

[0043]

[Table 1]

[0044]

[Table 2]

In Table 2, no. Nos. 19 to 27 are examples satisfying the specified requirements of the present invention. 1-18 and N
o. Comparative materials 28 to 36 lack any of the requirements defined in the present invention.

In Table 1, Example steels a to i are steel types q
(JIS SCM420 steel) and steel type r (JIS SCr
No significant difference is observed in the hardness after normalization as compared to the steel type 420 (steel 420), but in the steel type j with too much Si content, the steel type m with too much Mn content, and the steel type p with too much Mo content, Si + Mn + Mo] exceeds 2.20%,
Also, the steel type n having an excessively high Cr content has a very high hardness after normalizing.

In the comparative materials shown in Table 2, the steel material contained too much Si. Nos. 10, 28 and Nos. 14,
In the case of No. 32, the carbonitriding property was inhibited and [C
s] and [Ns] are less than the prescribed values, and [Cs] of 0.7% or more is secured in the other 32 cases. In the case where [Ns] is nitrided to 0.6% or more, the Si-based carbonitride amount is secured to 0.05% or more.

Then, using steel types a to i with appropriate chemical components of the steel material, [Cs] is 0.7% or more, [Ns]
No. of the embodiment in which 0.6% or more was secured by 19-2
In No. 7, the R value is also secured to 7.5 or more, and the hardness after tempering shows a high value of Hv 760 or more.

However, among the comparative examples in which the chemical components are inappropriate or the [Ns] is less than 0.6%, those having an R value of less than 7.5 have hardness after tempering satisfying Hv750. Not a low value. In addition, even if the R value is 7.5 or more, the steel material has too much Mn content. 13, 31,
No. and Mo amount are too large. In Nos. 16 and 34, the amount of retained austenite is excessive, and the hardness after tempering is extremely low. In addition, in the case of No. 14,3
In No. 2, a number of coarse precipitates and an incompletely quenched structure were observed in the surface hardened layer and the inner matrix in the structure observation after quenching, and the hardness after tempering was poor.

Table 3 shows that various test steels shown in Table 1 were subjected to normalizing treatment after hot forging, machined into gears, and subjected to the above carbonitriding treatment (A) or (B). Thereafter, a shot peening process with an arc height of 0.6 mmA was performed, the surface was ground by 0.1 mm, and the surface was subjected to a gear pitching test to evaluate the strength characteristics. The test conditions are as follows. Further, the carbonitriding treatment (A) was performed only on steel types a to j which are the example steels.

[Gear Pitching Test Conditions] Test gear shape: helical gear (module: 3.87,
Pitch circle diameter: 84 mm, number of teeth: 21) ・ Mating gear shape: helical gear (module: 3.87,
Pitch circle diameter: 116 mm, number of teeth: 29) ・ Surface pressure: 2.2 GPa ・ Relative sliding speed: 1.2 m / s ・ Oil temperature: 120 ° C ・ Pitching judgment: Vibration value is stable (± 0.05G) -Evaluated by the number of repetitions at the point of time exceeding +0.1 G ・ Number of test discontinuations: 20 million times

[0052]

[Table 3]

In Table 3, no. Nos. 46 to 54 are examples satisfying all the specified requirements of the present invention. Nos. 37 to 45 and Nos. 55 to 63 are comparative examples, in which the chemical composition of the steel material is within the specified range, [Cs] is 0.7% or more, [Ns] is carbonitrided to 0.6% or more, and the R value is also 7.5. No. In Examples 46 to 54, the pitting life was all 1
In contrast, the pitting life is less than 12 million times in the comparative examples in which the R value is less than 7.5 while the number is 4 million times or more. In addition, the carbonitriding property was inhibited by the inappropriate steel material components. 55 and No. In the case of No. 59, plastic deformation occurred at the time of the test, and evaluation was not possible.

[0054]

The present invention is configured as described above, and controls the chemical composition of the steel material appropriately, and determines the [Cs], [Ns] and R value of the surface hardened layer formed by carbonitriding. By appropriately controlling, or further specifying the content of Si-based carbonitride, the softening resistance of the surface layer is increased, and in particular, the resistance to fatigue fracture at the surface starting point is increased, and the pitting resistance is excellent. It has become possible to provide carbonitrided high-strength parts.

[Brief description of the drawings]

FIG. 1 is a diagram showing carbonitriding and quenching conditions adopted in Examples.

FIG. 2 is a view showing other carbonitriding and quenching conditions adopted in Examples.

Continued on the front page (72) Inventor Satoshi Abe 2 Nadahama-Higashi-cho, Nada-ku, Kobe City Inside Kobe Steel Co., Ltd.Kobe Steel Works (72) Inventor Hiroshi Kuramoto 2nd Nadahama-Higashi-cho, Nada-ku, Kobe City Kobe Steel Works Kobe Steel Works, Ltd. (72) Inventor Yoichi Watanabe 2 Takaracho, Kanagawa-ku, Yokohama, Kanagawa Prefecture Inside Nissan Motor Co., Ltd.

Claims (6)

[Claims] 1. C: 0.15 to 0.25% (mass%, the same applies hereinafter), Si: 0.40 to 0.9%, Mn: 0.05 to 0.7%, Cr: 1.25 From 2.55%, Mo: 0.35-1%, Al: 0.02-0.06%, and N: 0.007-0.015%, with the balance being substantially Fe [Si + Mn +
Mo] amount of 1.0 to 2.20%, having a surface hardened layer that has been quenched or tempered after carbonitriding or carbonitriding,
The C amount [Cs] from the surface to 0.1 mm is 0.7% or more, the N amount [Ns] is 0.6 to 2.0%, and the R value obtained by the following equation is 7.5 or more. A carbonitrided part characterized by the above. R value = 1.11 × [Cs] + 1.25 × [Ns] + 1.89 × Si + 1.22
× Mn + 0.67 × Mo + 3.94 2. The carbonitrided component according to claim 1, wherein the Si-based carbonitride is present in the surface hardened layer in an amount of 0.05% or more. 3. The steel according to claim 1, wherein, as another element, Cu: 1%
The carbonitrided part according to claim 1, wherein the carbonitrided part contains 1% or less and / or Ni: 1% or less (both do not include 0%). 4. The steel according to claim 1, further comprising Nb:
The composition according to any one of claims 1 to 3, comprising at least one selected from the group consisting of 1% or less, Ti: 1% or less, and B: 0.1% or less (both not including 0%). Of carbonitrided parts. 5. The steel according to claim 1, further comprising S, C as another element.
The carbonitrided component according to any one of claims 1 to 4, wherein the component contains at least one element selected from the group consisting of a, Zr, Sb, Pb, and Bi in an amount of 0.1% or less. 6. The carbonitrided part according to claim 1, which has been subjected to a shot peening treatment.